The invention is directed to luminescent materials based on cadmium borate activated with manganese, as well as a process for producing them.
Luminescent materials are solid substances which are capable of emitting light after absorbing energy (e.g., daylight, ultraviolet light, rontgen rays (X-rays), or corpuscular rays), which emitted light generally exhibits a greater wave length than that of the absorbed ray. The light radiation occurs during the excitation itself and at various lengths of time thereafter, which can be from milliseconds to hours.
Luminescent materials find expanding use, such as, for example, in light engineering to increase the energy yield in mercury lamps, in X-ray diagnostics, or in the coating of television tubes to make visible the modulated electron beam. Besides, they are used as coatings for ceramic products, especially in glazes for ceramic tiles.
Only a few inorganic compounds are capable of luminescence. Generally, the luminescence capability is first acquired through an activation, for example, by building small amounts of ions (activators) foreign to the crystal lattice into the crystal lattice (host lattice) of the inorganic compound (base material). Therefore, in the activator containing luminescent materials, it is a matter of crystallized compounds in which a few cations of the base materials are replaced by foreign cations. The built-in cations with the surrounding band of anions thereby has been conceived as the so-called luminescent centers which are responsible for the characteristic absorptions and emissions.
As base materials for the luminescent materials of significance, above all are salts of the alkaline earths and their side group elements zinc and cadmium as phosphates, silicates, borates, aluminates, and oxides. As activators, there are preferably used the rare earths and heavy metals, such as manganese, lead, tin, or antimony.
The luminescent materials generally must be very pure and have a crystal lattice as free as possible from disturbances since impurities in the starting materials act as quench centers and reduce the luminescent intensity of the luminescent materials. Lattice defects act similarly. The luminescent capability is damaged in the mechanical comminution of the compounds and in the extreme case can be completely destroyed.
The previously known luminescent materials have the further disadvantage that most of them are not temperature stable and disintegrate in many fritted glasses. Therefore, they are not suitable as pigments in glazes for ceramic tiles which require firing temperatures of over 700.degree. to 800.degree. C.
Luminescent materials based on cadmium borates activated with manganese are known. Cadmium oxide (CdO) forms a large number of cadmium borates with boron oxide (B.sub.2 O.sub.3), which borates upon activation with manganese result in green to red emitting luminescent materials depending on their composition. Only (2CdO.B.sub.2 O.sub.3 :Mn) has found practical use which upon excitation with mercury vapor radiation of 254 nm has an emission band in the orange red region at 626 nm. The luminescent material can be produced from the very pure starting compounds CdO or CdCO.sub.3, H.sub.3 BO.sub.3, and MnCO.sub.3 by calcining at 800.degree. C. but is very sensitive to impurities, mechanical comminution, high temperatures, and glass fluxes such as occur in the glazing of ceramic products.
Therefore, it was the problem of the present invention to develop luminescent materials based on manganese activated cadmium borate which can be produced at practically the same light intensity in the red region from commercial starting components having a lesser degree of purity, which are not sensitive to grinding and are as temperature stable and chemically stable to ceramic glazes as the known luminescent materials. Besides, there should be found a process for the production of these luminescent materials.